The invention pertains to annular chambers having two coaxial shells, wherein the inner shell delimits a channel traversed by a gaseous flow and the space between the two shells is divided by means of partitions into distinct cavities which communicate with the flow channel through openings or vents, each volume thus acting as a Helmholtz resonator which absorbs a portion of the acoustical energy in a given frequency band. Such chambers are notably used as housing walls in jet engines--such as turbojets--where numerous sources of noise are present together, including the combustion chamber and the blades of the compressor and turbine stages.
Such chambers are already known. They are of two kinds. In some of them, the cavities communicate directly with the channel by means of openings or slits provided in the inner shell. In other words, the opening of each resonator is a simple diaphragm, and the most important frequency tuning parameter that can be varied is the volume of each cavity. In the others, the vent is a duct. In order to change the absorption frequency and bandwidth, one may then more sensibly adjust its length and cross-sectional area. Nevertheless, even for chambers of the latter kind, there is the risk of being led to a significant lateral bulk if one attempts to tune to low frequencies; moreover, their construction is complex. See for example U.S. Pat. No. 2,765,044.